My question is that if I'm taking quite a few unguided images that I want to stack, at some/what point do I need to recenter the milky way in the viewfinder so the images will stack with more accuracy?

If this is the case, What do I use as a reference point when I recenter the image.

I wouldn't worry about it unless you are planning to take hundreds. Unguided images will be short (10 seconds or so to have perfectly round stars) meaning you could take 30 to stack in under 5 minutes. In 5 minutes, the stars won't move that much and the stacking program should have no problem finding enough stars to stack all your images.

Rhinoict... What I do is: I take ten shots and move slightly in the direction the stars are moving. If you can focus on a bright star, try to move it to the same place. When you stack, the outside of the pic will get a little crazy, but the meat of the pic should be go.